This invention relates to the suppression of electro-magnetic interference (EMI) using solid state components.
Many power supply systems generate EMI at unacceptable levels, requiring additional elements for the purpose of suppressing or removing the generated EMI. For example, a switching power supply plugged into a conventional power outlet generates RF noise as its power switch cycles on and off repeatedly. To the power outlet, this RF noise is unwanted xe2x80x9cpollution xe2x80x9d that will interfere with the operation of other nearby electronic components. As a result, manufacturers of electronic components often must certify that their products"" EMI levels fall below maximum acceptable limits, i.e., satisfy electromagnetic compatibility (EMC) standards.
Given that many components unavoidably generate EMI, it is a common practice to introduce an EMI filter between such components and the AC power lines so as to satisfy EMC standards. EMI filters generally employ reactive elements (capacitors or inductors). Accordingly, conventional EMI filters may incorporate common-mode chokes, differential chokes, line-to-line or across-the-line X capacitors, and line-to-ground Y capacitors. Regardless of the type of EMI filter used, the desired result is a filter whose transfer function suppresses the expected noise spectrum in the frequency domain. This can be quite difficult as the noise spectrum can vary with conditions such as application load, input power and voltage levels, temperature, component age, PCB layout, switching frequency, power supply topology, etc. The difficulty of EMI filter design is compounded by the expense of the EMI filter components. Capacitors and inductors are inherently expensive, and their energy storage capabilities are not easily reproduced with inexpensive solid-state components.
Consequently, what is needed is an alternative to an EMI filter, built of inexpensive solid-state components that can be incorporated with minimal design effort into any power supply application.
A high impedance insertion system incorporating a bi-state device having a high-impedance state and a low-impedance state couples between a power source and a load capable of producing EMI. By cyclically alternating between the high-impedance state and the low-impedance state, the system blocks load-produced EMI from coupling to the power source.
In accordance with one embodiment of the invention, the high impedance insertion system includes a full-wave rectifier circuit for receiving an AC input from an AC main power source. Depending on the instantaneous value of the AC input voltage, the rectifier circuit cyclically alternates between being forward biased and being reversed biased. When the rectifier circuit is forward biased, a low impedance path permits EMI from the load to be conductively coupled to the AC mains. Conversely, when the rectifier circuit is reversed biased, high impedance between the load and the AC mains prevents conductive coupling of EMI to the AC mains. During the periods of time when the rectifier circuit is forward biased, the high impedance insertion system places the bi-state device in the high-impedance state, thus blocking EMI from being transmitted to the AC mains.